RAMALLAH, West Bank — Palestinians will hold presidential and legislative elections by September, a top aide to President Mahmoud Abbas announced Saturday, a surprise move apparently prompted by the political unrest spreading in the Arab world.

Abbas aide Yasser Abed Rabbo did not give a firm date for elections, but said the chief Palestinian decision-making body, the Palestine Liberation Organization, was already making preparations.

“We call on parties to put aside all of their differences and to focus on conducting the elections by September at the latest,” he told a news conference.

However, Abbas’ main political rival, the Islamic militant Hamas, said it would not participate.

Abed Rabbo spoke a day after Egyptian President Hosni Mubarak stepped down in response to nearly three weeks of mass protests against his 30-year rule.

Palestinian elections were meant to be held last year, but the vote was put off because of the split between Abbas’ government in the West Bank and the rival Hamas regime in the Gaza Strip.